1. Field of the Invention
The present invention relates generally to a method for forming an SOI (Silicon-on-Insulator) film in which a monocrystalline silicon film layer serving as an active region is formed on an insulating film.
2. Description of the Prior Art
In a semiconductor integrated circuit device, a method for achieving high integration density is roughly divided into two methods. One method, which has been conventionally used extensively, is to improve the integration degree of semiconductor devices formed on one chip by making fine structure of many semiconductor devices formed on the surfaces of a semiconductor substrate and isolation structure of each of the semiconductor devices. On the other hand, the other method is to improve the integration degree by stacking semiconductor devices respectively through an insulating layer on the surface of a semiconductor substrate to implement so-called three-dimensional device structure. An important technique for implementing the three-dimensional device structure includes a technique for forming an SOI film. This technique is used for forming an insulating film on the surface of a silicon layer having semiconductor devices formed therein and forming a new monocrystalline silicon film (referred to as an SOI film) on the surface of the insulating film. A new semiconductor device is formed on the monocrystalline silicon film layer, so that three-dimensional device structure having semiconductor devices stacked therein can be implemented.
The method for forming the SOI film includes a method referred to as an SIMOX (Separation by Implanted Oxygen) process, which is described in an article by Katsutoshi Izumi, et al., entitled "High Speed C-MOS IC Using Buried SiO.sub.2 Layers Formed by Ion Implantation", Japanese Journal of Applied Physics, Volume 19 (1980), Supplement 19-1, pp. 151-154. Referring now to FIGS. 5A and 5B, the SIMOX process will be described.
First, as shown in FIG. 5A, oxygen ions 2 are implanted into the surface of a silicon substrate 1 using an ion implantation process. As conditions of ion implantation, acceleration energy is selected such that the oxygen ions 2 can reach a depth at which an embedded oxide film layer is to be formed and a dose is selected such that the embedded oxide film layer formed by the implanted oxygen ions 2 has satisfactory insulating characteristics. In this example, the oxygen ions 2 are implanted at an implantation energy of 150 KeV and at a dose of 1.2.times.10.sup.18 cm.sup.-2.
Then, as shown in FIG. 5B, the silicon substrate 1 having the oxygen ions 2 ion-implanted thereinto is heat-treated at a temperature of 1150.degree. C. for two hours, to form an embedded oxide film layer 3. In this example, the depth D.sub.pt of the embedded oxide film layer 3 is 370 nm from the surface of an SOI film 4 formed by separating the silicon substrate 1. Thus, the monocrystalline silicon film layer 4 of an SOI film is formed on the surface of the silicon substrate 1 through the embedded oxide film layer 3 serving as an insulating layer.
However, in the conventional SIMOX process, since the embedded oxide film layer 3 is formed by heat-treating the oxygen ions ion-implanted into the silicon substrate 1 to oxidize the same, the amount of the oxygen ions 2 to be implanted must be large, i.e., approximately 10.sup.18 cm.sup.-2. Consequently, a crystal lattice within the silicon substrate 1 is deformed due to damage caused by ion implantation of oxygen ions. The damage can not be completely recovered by heat treatment for forming the embedded oxide film layer 3. Thus, in the SIMOX process, it is difficult to form the SOI film 4 having good quality and uniform monocrystalline structure. Therefore, conventionally, it has been necessary to form an epitaxial layer of new monocrystalline silicon on the surface of the SOI film 4 further using a CVD (Chemical Vapor Deposition) process and to form a new device on the epitaxial layer.
In addition, the method for forming the SOI film further includes a method using a solid phase epitaxial growth process shown in FIGS. 6 and 7. The solid phase epitaxial growth process is described in, for example, an article by Hiroshi Yamamoto, et al., entitled "Enhancement of Lateral Solid Phase Epitaxial Growth in Evaporated Amorphous Si Films by Phosphorus Implantation", Appl. Phys. Lett., Vol, 46, No. 3.1, Feb. 1985, pp. 268-270. In this process, as shown in FIG. 6, an oxide film 5 is formed on the surface of a silicon substrate 1. An opening 6 reaching the surface of the silicon substrate 1 is formed in a part of the oxide film 5. The opening 6 is referred to a seed portion. In addition, an amorphous silicon layer 7 is formed on the surface of the oxide film 5. A method for forming the amorphous silicon layer 7 includes a method for depositing amorphous silicon using a CVD process and a method for depositing polysilicon (polycrystalline silicon) using a CVD process and then, ion-implanting silicon ions to obtain amorphous polysilicon. Then, the amorphous silicon layer 7 is heat-treated at a temperature of approximately 600.degree. C. to recrystallize of the amorphous silicon layer 7, to form an SOI film 4 the monocrystalline silicon film. In this recrystallizing process, solid-phase epitaxial growth of the amorphous silicon layer 7 deposited in the opening (seed portion) 6 of the oxide film 5 is first made with a surface region of the silicon substrate 1 as a nucleus. When growth is further continued, the solid phase epitaxial growth is continued from the neighborhood of the seed portion 6 toward a plane region of the silicon layer 7. This continuous solid phase epitaxial growth is interrupted by irregular nucleation of the silicon layer 7, to be restricted in a neighboring region of the seed portion 6. More specifically, as shown in FIG. 7, a region of the monocrystalline silicon layer 4 growing from the seed portion 6 formed in the oxide film 5 is restricted to a region L at a distance of approximately several .mu.m to 20 several .mu.m from the seed portion 6. Thus, in this solid phase epitaxial process, a region where the SOI film can be formed is restricted to the neighboring region of the seed portion. In addition, in a region of the seed portion, the silicon substrate 1 and the SOI film 4 are conducted by monocrystalline silicon, so that this region can not be employed as a region where semiconductor devices are formed.
As described in the foregoing, in the conventional solid phase epitaxial process, a considerably wide monocrystalline silicon layer region can not be formed. Thus, it is difficult to arrange semiconductor devices in a very small monocrystalline silicon layer region.